The principles of the silver salt diffusion transfer process are described in U.S. Pat. No. 2,352,014 and other references. An imagewise exposed silver halide emulsion layer is either disposed in contact or brought into contact with an image receiving layer in the presence of a developing agent and a silver halide solvent, so that unexposed silver halide is converted to a soluble silver complex salt. In the exposed part of the silver halide emulsion layer, the silver halide is developed into silver, which is unable to dissolve any further and hence is incapable of diffusion. In the unexposed area of the silver halide emulsion layer, the silver halide is converted to a soluble silver complex salt, which is transferred to the image receiving layer where the complex salt forms a silver image usually in the presence of nuclei for physical development.
Silver salt diffusion transfer techniques find extensive use in reproduction of documents, making of lithographic printing plates, preparation of block copy materials and in instant photography (for a review of these applications, see A. Rott and E. Weyde, Photographic Silver Halide Diffusion Transfer Processes, Focal Press, London, 1972). For the purpose of accurately reproducing information carried by the original, it is important to produce a silver image of high maximum density, contrast and good definition in the image receiving layer for silver salt diffusion transfer. Being capable of providing high contrast, the silver salt diffusion process is highly adapted to the formation of a line image or a contact-screen halftone image.
Conventionally, most silver salt diffusion transfer techniques use a solution containing a developing agent and a silver halide solvent in order to process a light-sensitive material containing a silver halide emulsion layer and an image receiving material having an image receiving layer containing nuclei for physical development. This is gradually being replaced by a more recent and popular method wherein a light-sensitive material containing a developing agent and an image receiving material containing nuclei for physical development are processed by a solution which contains a silver halide solvent but which is substantially free of a developing agent.
In the new method, the processing composition contains no developing agent and its pH can be increased to a level that permits a significantly high transfer activity, and hence a faster transfer speed. However, if a processing composition having high pH is left in a diffusion transfer processor having a relatively large opening, the pH of the composition easily drops due mainly to the absorption of carbon dioxide in the air. The pH of the processing composition will also be decreased when it is used to process a photographic material on account of either the development reaction or the dissolution of an acid substance or the like from the light-sensitive material into the processing composition. In whichever case, the development activity of the processing composition is reduced in a manner that adversely affects the photographic performance of the processed material in respect to the maximum density, gradation and stain.
In order to ensure rapid development, the processing solution is usually rendered highly alkaline, i.e., at a pH in the range of about 10 to 14. For this purpose, an alkali metal salt of phosphoric acid such as trisodium phosphate (said salt is hereunder simply referred to as a phosphate salt) is preferably used.
However, as shown in Japanese Patent Application (OPI) No. 24349/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a phosphate salt used in a fairly large amount is not necessarily favorable to rapid development because, firstly, crystallization occurs easily, and, secondly, an excessively high ionic strength inhibits the swelling of a colloidal substance such as gelatin. Furthermore, the present inventors confirmed by experiments that when a processing composition containing a fairly large amount of the phosphate salt was used some time after its preparation, the transferred image had a reduced gradation or stain occurred within the image.
A substituted aminoalcohol represented by the formula: R--NH--C.sub.2 H.sub.4 OH (R: lower alkyl group having 1 to 4 carbon atoms) is incorporated into a processing solution for the silver salt diffusion transfer process in an amount which usually ranges from about 5 to 20 ml per liter of the solution. This is effective in providing a faster transfer speed, higher contrast and better sharpness, but as is stated in Japanese Patent Application (OPI) No. 79445/80 (corresponding to U.S. Pat. No. 4,362,811), this compound accelerates the deposition of silver and causes an undesired decrease in the transfer density.